This invention relates generally to nanoparticles and nanostructures and specifically to a method for coating nanoparticles on nanostrustured surfaces and systems and devices fabricated by the method of this invention.
Metal (e.g., Ag, Au and Cu) nanoparticles can exhibit a strong optical absorption and reflectance in the UV-visible range of the electromagnetic spectrum that is not present in the spectrum of the bulk metal. When the particle size is much smaller than the wavelength of the incident electromagnetic radiation, the electrons in the particle can move in phase which can result in their generating a giant oscillating dipole (or multipole depending on the shape of the particle). This collective motion of the electrons generates surface polarization charges on each side of the particle, which act as a restoring force on these electrons. Because of the restoring force a resonance condition occurs at a certain frequency at which the amplitude of the oscillating dipole can be excited to a maximum. These collective electron oscillation modes are termed particle plasmons, localized surface plasmons or simply surface plasmons. (see P. Mulvaney, MRS Bulletin 26, 1009 (2001).)
These plasmon modes can be excited by electromagnetic radiation (photon) absorption. They can also be excited by high energy electron collisions. The associated absorption of the electromagnetic energy (photons) for the generation of these collective electron oscillation modes is seen as a band in the optical absorption spectrum and is called the surface plasmon absorption band. It is well established that plasmon energy of particles (e.g., the wavelength or frequency of the surface plasmon absorption peak) is dependent upon size, shape, and interparticle spacing as well as on particle and local environment dielectric properties. When the particle size increases (or when nanoparticles aggregate so electrons can travel from one nanoparticle to another) electrons become dephased and they cannot generate a strong restoring force. Consequently, the plasmon absorption band broadens and gets weaker.
Metal nanoparticles also exhibit interesting electronic, magnetic and catalytic properties that are not present in the bulk metal. These unique properties of nanoparticles are tunable by varying particle size, shape and spacing. For example, colloidal suspensions of gold can be red, purple, or blue, depending on the size, spacing and environment of the gold particles. In addition, gold clusters on a titania surface have been shown to yield catalytic oxidation of carbon monoxide that becomes most effective when the cluster diameter is reduced to about 3 nm. Hence, metal nanoparticles or nanostructured metal surfaces with their unique properties and enhanced surface area offer exciting opportunities for the development of novel sensors and detectors, catalysts, and absorbing and adsorbing media.
Nanoparticle Surface Plasmon Resonance (SPR) spectroscopy is a sensing/detection application based on exploiting the interaction of plasmons with electromagnetic radiation. SPR has been extensively used to monitor a broad range of analyte-surface binding interactions including the adsorption of small molecules, ligand-receptor binding, protein adsorption, antibody-antigen binding, DNA and RNA hybridization and protein-DNA interactions. (see A. J. Haes and R. P. Van Duyne, J. Amer. Chem. Soc. 124, 10596 (2002).) Heretofore, the sensing mechanism of SPR spectroscopy has been focused on using the shift in SPR energy of a noble metal nanoparticle that occurs in response to a change in the surrounding refractive index. (See A. J. Haes and R. P. Van Duyne, J. Amer. Chem. Soc. 124, 10596 (2002)). See also C. Sönnichsen, S. Geier, N. E. Hecker, G. von Plessen, J. Feldmann, H. Ditlbacher, B. Lamprecht, J. R. Krenn, F. R. Aussenegg, V. Z-H. Chan, J. P. Spatz, and M. Möller, Appl. Phys. Lett. 77, 2949 (2000)). These surrounding-medium index of refraction changes are due to events such as analyte binding at or near the nanoparticle surface. The refractive index sensitivity of SPR biosensors and chemosensors based on analyte binding at or near the nanoparticle surface has been reported to be on the order of 1 part in 105-106 corresponding to an areal mass sensitivity of 10-1 pg/mm2.
Another plasmon-electromagnetic interaction that can be exploited for sensing and detection is surface enhanced Raman scattering. It offers detailed information on molecules, which is a highly demanded capability in various disciplines such as analytical chemistry, molecular biology, pharmacology, nanotechnology, homeland security, and environmental science. In particular, observation of physical and chemical effects in a single molecule is of basic scientific interest and can provide insight into the individual properties of the molecules. Such insight can be lost or masked in ensemble-averaged measurements. In the past decade, fluorescence spectroscopy has proven to be a useful technique for probing single molecules. However, the broad fluorescence bands are relatively insensitive to molecular structure. Furthermore, because the excitation energy is in resonance with electronic transitions during fluorescence, probed molecules rapidly undergo photodecomposition, reducing the time during which a molecule is available for fluorescence detection. Also the above mentioned SPR shifts lack detailed information; i.e., an SPR shift only shows that some binding event at or near the nanoparticle surface or some absorption has occurred
On the other hand, surface-enhanced Raman scattering (SERS) has emerged as one of the most sensitive spectroscopic methods available for the detection of a wide range of adsorbate molecules down to the single molecule detection limit. (See K. Kneipp, H. Kneipp, I. Itzkan, R. R. Dasari, and M. S. Feld, Chem. Rev. 99, 2957 (1999). The use of Raman scattering to investigate adsorbates on surfaces was initially thought to be of insufficient sensitivity due to the extremely small cross sections associated with this effect (−10−30 cm2 per molecule which is 14 orders of magnitude smaller than that of a fluorescent dye molecule in fluorescence). However, this situation can be dramatically improved in SERS where the cross sections can be enhanced up to 1014-1015 times. (See S. Nie and S. R. Emory, Science 275, 1102 (1997)—K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari, and M. S. Feld, Phys. Rev. Lett. 78, 1667 (1997)) SERS simply involves spectral measurement of the Raman scattered light from an analyte material (e.g., molecules) adsorbed on or attached to metal nanoparticles (or vice versa) of subwavelength dimensions. In SERS, the frequency of the incident as well as the scattered light is around that of the surface plasmons in the metal nanoparticles. Once the surface plasmon modes are excited and in resonance with the impinging electromagnetic field such as an impinging laser beam, strong dipoles are induced in the metal nanoparticles that in turn develop strong local electric fields (associated with electromagnetic field) in the vicinity of the nanoparticle surfaces. This region of strong local electric fields overlaps the location of the molecules adsorbed or bonded on or near the nanoparticle surfaces. This amplification in local electric field is for both the incident light and Raman scattered light. Consequently, to a first approximation the enhancement in Raman scattering caused by the adsorbed molecules varies as the fourth power of the attenuation in incident electric field (See K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari, and M. S. Feld, Phys. Rev. Lett. 78, 1667 (1997).) Therefore, enormous gains are possible in the intensity of the SERS signal and, therefore, enormous increases in the fingerprinting capability for molecules in the overlap volume.
Another advantage of SERS is that, unlike in fluorescence, photodecomposition of probed molecules can be avoided in SERS since the incident light is not required to be in resonance with the molecular transitions causing the Raman scattering. Furthermore, since the SERS vibrational spectrum contains detailed structural information about the molecule, the molecular fingerprinting can be very specific. Finally, because vibrational states have shorter lifetimes than electronic ones, Raman scattered radiation is more intense than fluorescence by about a factor of 103 under saturation. (See K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari, and M. S. Feld, Phys. Rev. Lett. 78, 1667 (1997).).
Nie and Emory (Science, 275, 1102 [1997]), and Kneipp et al. (Phys. Rev. Lett. 78, 1667 [1997]) independently achieved the first demonstration of single molecule detection by SERS that involved enhancement factors of about 1014. Both groups detected single dye molecules attached to colloidal silver particles in an aqueous solution. Alternative SERS practice demonstrated by others is to use substrates coated with metal nanoparticles rather than solutions with nanoparticles. (See M. Moskovits, Rev. Mod. Phys. 57, 783 (1985); see also H. Seki, J. Vac. Sci. Technol. 18, 633 (1981).) The advantage of this approach lies in the immobilization of metal particles on the substrate surface. Hence, unlike in the case of colloidal particles suspended in an aqueous solution, complications arising from thermal motion are out of the question. Consequently, immobilization makes it easier to probe a molecule attached on a nanoparticle. Furthermore, aggregation of nanoparticles can be avoided with proper immobilization, as we demonstrate in this invention. Up to now, the substrates for SERS immobilization have been prepared by, for example, evaporation or sputtering of metal island (ultrathin) films, electrochemical roughening of metal surfaces, or patterning of continuous metal films by electron beam lithography.
The most common and simple approach to the synthesis of metal nanoparticles is through colloidal chemistry. This method involves a precipitation reaction in a liquid solution to form the nanoparticles; however, the particles may aggregate unless an appropriate surfactant is used. On the other hand the surfactant may hinder molecular adsorption on the particle surface. Therefore, the surfactant may prevent certain sensing mechanisms if the particles are to be used for sensing. Similarly, the surfactant will degrade the catalytic activity of the nanoparticles. Furthermore, if colloidal particles are desired to be disposed and immobilized on a substrate, it is difficult to obtain nanoparticle arrays at desired uniformity, density, and dispersion by using colloidal nanoparticles.
It has been shown that the solution chemistry of colloidal nanoparticles can be avoided. Specifically, it has been shown that metal nanoparticles can be directly synthesized on a surface by contact of the surface to a metal salt, if that surface material serves as reducer for the said metal salt. This direct synthesis has been demonstrated only for continuous single crystal (non-porous) surfaces of silicon and germanium. Synthesis of nanoparticles has been shown to proceed on Si surfaces only in the presence of HF. The HF is required to remove the oxide so that electron transfer can be maintained and metal deposition can occur. (L. A. Nagahara, T. Ohmori, K. Hashimoto, and A. Fujishima, J. Vac. Sci. Technol. A 11, 763 (1993)) Because reduction proceeds at the expense of oxidation of Si, HF is needed for the dissolution of the oxide, which otherwise hinders the electron transfer. On the other hand, the continuous (flat) Ge surfaces have been found to support electroless growth of noble metal nanoparticle using pure metal salt solutions without HF. (See L. A. Porter, H. C. Choi, A. E. Ribbe, and J. M. Buriak, Nano Lett. 2, 1067 (2002); see also L. A. Porter, H. C. Choi, J. M. Schmeltzer, A. E. Ribbe, L. C. C. Elliott, and J. M. Buriak, Nano Lett. 2, 1369 (2002)) This is easily understood since Ge oxide dissolves in water. (Porter et al.) However, this approach of synthesizing metal nanoparticles on a flat (single crystal) surface offers no control over particle size, spacing and dispersion. An alternative approach to try to solve these deficiencies has been tried. It uses the patterning of continuous metal films by electron beam lithography to form nanoparticles immobilized on a substrate with precise control of size, separation and shape. (See L. Gunarsson, E. J. Bjerneld, H. Xu, S. Petronis, B. Kasemo, and M. Käll, Appl. Phys. Lett. 78, 802 (2001)) However, this technique involves high costs, low throughput and is limited to a minimum particle size of −1 Onm. Metal nanoparticles can alternatively be obtained by condensation of a metal vapor on a substrate surface in the form of islands. (See H. Seki, J. Vac. Sci. Technol. 18, 633 (1981)) Although vapor deposition is a lower cost and higher throughput process than electron beam lithography, control of particle size and spacing is difficult.
It is an object of this invention to provide a method for synthesizing and immobilizing contaminant-free/surfactant-free metal nanoparticles on a surface with controlled particle size, dispersion, spacing, and density.
It is also an object of the present invention to provide the said method for the fabrication of high-sensitivity, low-background SERS/SPR-sensor/detector devices and systems, and catalytic/absorption/adsorption-sensors/detector devices and systems.
It is yet another object of this invention to provide inexpensive and high-throughput methods for the fabrication of the above referenced devices.
It is a further object of this invention to provide a non-vacuum-based as well as non-colloidal chemistry based method of synthesizing metal nanoparticles.
It is also an object of this invention to provide a method for synthesizing metal nanoparticles that does not require any reducing agents, catalysts, or coating agents to form the metal nanoparticles.